How fast does a black hole spin? And how does it matter? An international team including Lawrence Livermore National Laboratory scientists has definitively measured the spin rate of a supermassive black hole for the first time. The findings, made by the two X-ray space observatories, NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton, solve a long-standing debate about similar measurements in other black holes and will lead to a better understanding of how black holes and galaxies evolve.

ADVERTISEMENT

A black hole is a region of space time from which gravity prevents anything, including light, from escaping. As a result a black hole has little to interact with in terms of the outside universe. It possesses gravity, may have a charge and it may spin or rotate.

These properties are special because they are visible from outside a black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole. Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

"We can trace matter as it swirls into a black hole using X-rays emitted from regions very close to the black hole," said Fiona Harrison, NuSTAR principal investigator at the California Institute of Technology, Pasadena, and coauthor of a new study appearing in the Feb. 28 edition of Nature. "The radiation we see is warped and distorted by the motions of particles, and by the black hole's incredibly strong gravity."

Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the space time immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole, this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.

The formation of supermassive black holes is thought to mirror the formation of the galaxy itself, since a fraction of all the matter drawn into the galaxy finds its way into the black hole. Because of this, astronomers are interested in measuring the spin rates of black holes in the hearts of galaxies.

The observations also are a powerful test of Einstein's theory of general relativity, which holds that gravity can bend light and space-time. The X-ray telescopes detected these warping effects in the most extreme of environments, where the immense gravity field of a black hole is severely altering space-time.

NuSTAR, a NASA Explorer-class mission launched in June of 2012, is uniquely designed to detect the highest-energy X-ray light in great detail. For Livermore, the predecessor to NuSTAR was a balloon-borne instrument known as HEFT (the High Energy Focusing Telescope) that was funded, in part, by a Laboratory Directed Research and Development investment beginning in 2001. NuSTAR takes HEFT's X-ray focusing abilities and sends them beyond Earth's atmosphere on a satellite. The optics design and the manufacturing process for NuSTAR are based on those used to build the HEFT telescopes.

"We know that black holes have a strong link to their host galaxy," said astrophysicist Bill Craig, a member of the LLNL team. "Measuring the spin, one of the few things we can directly measure from a black hole, will give us clues to understanding this fundamental relationship."

The team used NuSTAR to observe X-rays emitted by hot gas in a disc just outside the "event horizon," the boundary surrounding a black hole beyond which nothing, including light, can escape.

Previous measurements were uncertain because obscuring clouds around the black holes could, in theory, have been confusing the results. By working together with XMM-Newton, NuSTAR was able to see a broader range of X-ray energy, penetrating deeper into the region around the black hole. The new observations ruled out the idea of obscuring clouds, demonstrating that spin rates of supermassive black holes can be determined conclusively.

NuSTAR and XMM-Newton simultaneously observed the two-million-solar-mass supermassive black hole lying at the dust and gas-filled heart of a galaxy called NGC 1365. The results showed that the black hole is spinning close to the maximal rate allowed by Einstein's theory of gravity.

"These monsters, with masses from millions to billions of times that of the sun, are formed as small seeds in the early universe and then grow by swallowing stars and gas in their host galaxies, and/or merging with other giant black holes when galaxies collide," said Guido Risaliti, lead author of the new study from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. and the Italian National Institute for Astrophysics. "Measuring the spin of a supermassive black hole is fundamental to understanding its past history and that of its host galaxy."